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Full text of "Handbook Of Chemical Engineering - I"

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410                              CHEMICAL ENGINEERING
represents the true thermodynamic scale (as closely as we have any means whatever of determining this agreement) up to 1,100C., the highest temperature at which the resistance thermometer can be operated satisfactorily. If the scale denned in this manner is arbitrarily chosen as the working temperature scale, we have a means of measuring temperature far more precisely than is possible with the gas thermometer. If the boiling point of sulphur should be found at any time to differ slightly from the above accepted value, the scale defined by the platinum resistance thermometer could be altered by known small amounts to conform with the newly accepted value.
The gas thermometer has never been used at a temperature higher than 1,550C. Above 13500C. the temperature scale is defined by means of the Stefan-Boltzmann or Wien-Planck radiation laws. These laws have a theoretical significance, and experimental evidence is such as to lead to the conclusion that the scales defined by these two radiation laws are in mutual agreement and that they represent the ideal thermodynamic scale.
The provisional scale adopted by the Bureau of Standards may be expressed in terms of the following fixed points. On the basis of the true thermodynamic scale these standard temperatures are known to an accuracy of possibly 0.5 at 500C., and 3 at 1,200C. On the basis of the platinum resistance thermometer scale defined as above, the temperatures below 1,000C. can be determined with possibly 10 times this precision. The accuracy with which the platinum point is known on the thermodynamic scale is probably 10C., and the accuracy of the tungsten point may be estimated as 50C.
TABLE 1.FREEZING OR MELTING POINTS, DEGREES CENTIGRADE
Tin..................... 231.9       Gold...................   1,063
Lead.................... 327.3       Copper.................   1,083
Zinc.................... 419.4       Nickel..................   1,452
Antimony............... 630.0       Palladium...............   1,550
Aluminum............... 658.7       Platinum...............   1,755
Copper-silvereutectic..... 779.0       Alumina................  2,050
Silver................... 960.5       Tungsten...............  3,260
BOILING POINTS
Naphthalene........................ 217.96C. + 0.058(P - 760)1
Benzophenone...................... 305.9 C. + 0.063(P - 760)1
Sulphur............................ 444.6 C. + 0.090(P - 760)1
The above table applies for materials of the highest possible purity. The presence of impurities will, in general, lower freezing or melting points and raise boiling points.
HIGH-TEMPERATURE THERMOMETRY
Introduction.High-temperature thermometry deals with instruments used to measure temperature in the range 100 to 550C. This temperature range is more or less arbitrary, although the upper limit (550C.) is fairly definite and coincides with the approximate upper limit of use of the mercury-in-glass thermometers.2
In this range (100 to 550) the domains of pyrometry and thermometry overlap somewhat, for high-temperature resistance thermometers and thermocouples generally classed as pyrometers are often used below 550C. These instruments will not be
1P denotes pressure in mm. Hg.
2 Quartz glass thermometers filled with mercury under pressure have been constructed to measure temperatures up to 750C. They are not used in this country, however, except to a negligible extent. Reports as to their behavior have not been promising.